This is a regression against 5.2.0 (which didn't have this bug), due to
optimizations introduced in the stable branch after the release. The code path
for optimizing access to the members of C++ based singletons through the
regular meta-object properties would end up excluding access to enums when the
lookup happens at run-time. The run-time getter for the singleton itself would
return a wrapped QObject instead of a QQmlTypeWrapper, and only the latter
includes enums.

As QML based singletons (composite singletons) cannot declare enums, we can
continue to do fast lookups on these, but otherwise have to fall back to the
slower code path.